Re: [RFC PATCH net-next v6 08/15] page_pool: devmem support
From: Mina Almasry
Date: Tue Mar 05 2024 - 16:43:27 EST
On Mon, Mar 4, 2024 at 6:02 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> Convert netmem to be a union of struct page and struct netmem. Overload
> the LSB of struct netmem* to indicate that it's a net_iov, otherwise
> it's a page.
>
> Currently these entries in struct page are rented by the page_pool and
> used exclusively by the net stack:
>
> struct {
> unsigned long pp_magic;
> struct page_pool *pp;
> unsigned long _pp_mapping_pad;
> unsigned long dma_addr;
> atomic_long_t pp_ref_count;
> };
>
> Mirror these (and only these) entries into struct net_iov and implement
> netmem helpers that can access these common fields regardless of
> whether the underlying type is page or net_iov.
>
> Implement checks for net_iov in netmem helpers which delegate to mm
> APIs, to ensure net_iov are never passed to the mm stack.
>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>
Per David Howell's request, I'm forwarding these 2 patches from the
series to linux-mm & Mathew Wilcox.
https://lore.kernel.org/netdev/950858.1709622997@xxxxxxxxxxxxxxxxxxxxxx/
+linux-mm +Mathew Wilcox +David Howells
Some background on how we arrived at the current design may be useful.
This was largely discussed across 2 (long) email threads, largely with
Jason Gunthorpe and Christian Konig:
https://lore.kernel.org/lkml/CAHS8izPm6XRS54LdCDZVd0C75tA1zHSu6jLVO8nzTLXCc=H7Nw@xxxxxxxxxxxxxx/
https://lore.kernel.org/bpf/CAHS8izMdKYyjE9bdcFDWWPWECwVZL7XQjtjOFoTq5_bEEJvN6w@xxxxxxxxxxxxxx/
The approach is based on supporting dma-buf scatterlist entries in the
networking stack, to support receiving data directly into GPU memory
typically represented by dmabuf. I added the abstract netmem_ref type
in a recent series that can multiplex between struct pages and
non-struct pages (called net_iovs in this series):
https://lore.kernel.org/netdev/20240214223405.1972973-1-almasrymina@xxxxxxxxxx/
> ---
>
> v6:
> - Rebased on top of the merged netmem_ref type.
> - Rebased on top of the merged skb_pp_frag_ref() changes.
>
> v5:
> - Use netmem instead of page* with LSB set.
> - Use pp_ref_count for refcounting net_iov.
> - Removed many of the custom checks for netmem.
>
> v1:
> - Disable fragmentation support for iov properly.
> - fix napi_pp_put_page() path (Yunsheng).
> - Use pp_frag_count for devmem refcounting.
>
> ---
> include/net/netmem.h | 142 ++++++++++++++++++++++++++++++--
> include/net/page_pool/helpers.h | 25 +++---
> include/net/page_pool/types.h | 1 +
> net/core/page_pool.c | 26 +++---
> net/core/skbuff.c | 23 +++---
> 5 files changed, 171 insertions(+), 46 deletions(-)
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 21f53b29e5fe..8699788d587d 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -9,14 +9,51 @@
> #define _NET_NETMEM_H
>
> #include <net/devmem.h>
> +#include <net/net_debug.h>
>
> /* net_iov */
>
> +DECLARE_STATIC_KEY_FALSE(page_pool_mem_providers);
> +
> +/* We overload the LSB of the struct page pointer to indicate whether it's
> + * a page or net_iov.
> + */
> +#define NET_IOV 0x01UL
> +
> struct net_iov {
> + unsigned long __unused_padding;
> + unsigned long pp_magic;
> + struct page_pool *pp;
> struct dmabuf_genpool_chunk_owner *owner;
> unsigned long dma_addr;
> + atomic_long_t pp_ref_count;
> };
>
> +/* These fields in struct page are used by the page_pool and net stack:
> + *
> + * struct {
> + * unsigned long pp_magic;
> + * struct page_pool *pp;
> + * unsigned long _pp_mapping_pad;
> + * unsigned long dma_addr;
> + * atomic_long_t pp_ref_count;
> + * };
> + *
> + * We mirror the page_pool fields here so the page_pool can access these fields
> + * without worrying whether the underlying fields belong to a page or net_iov.
> + *
> + * The non-net stack fields of struct page are private to the mm stack and must
> + * never be mirrored to net_iov.
> + */
> +#define NET_IOV_ASSERT_OFFSET(pg, iov) \
> + static_assert(offsetof(struct page, pg) == \
> + offsetof(struct net_iov, iov))
> +NET_IOV_ASSERT_OFFSET(pp_magic, pp_magic);
> +NET_IOV_ASSERT_OFFSET(pp, pp);
> +NET_IOV_ASSERT_OFFSET(dma_addr, dma_addr);
> +NET_IOV_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
> +#undef NET_IOV_ASSERT_OFFSET
> +
> static inline struct dmabuf_genpool_chunk_owner *
> net_iov_owner(const struct net_iov *niov)
> {
> @@ -69,20 +106,27 @@ net_iov_binding(const struct net_iov *niov)
> */
> typedef unsigned long __bitwise netmem_ref;
>
> +static inline bool netmem_is_net_iov(const netmem_ref netmem)
> +{
> +#ifdef CONFIG_PAGE_POOL
> + return static_branch_unlikely(&page_pool_mem_providers) &&
> + (__force unsigned long)netmem & NET_IOV;
> +#else
> + return false;
> +#endif
> +}
> +
> /* This conversion fails (returns NULL) if the netmem_ref is not struct page
> * backed.
> - *
> - * Currently struct page is the only possible netmem, and this helper never
> - * fails.
> */
> static inline struct page *netmem_to_page(netmem_ref netmem)
> {
> + if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
> + return NULL;
> +
> return (__force struct page *)netmem;
> }
>
> -/* Converting from page to netmem is always safe, because a page can always be
> - * a netmem.
> - */
> static inline netmem_ref page_to_netmem(struct page *page)
> {
> return (__force netmem_ref)page;
> @@ -90,17 +134,103 @@ static inline netmem_ref page_to_netmem(struct page *page)
>
> static inline int netmem_ref_count(netmem_ref netmem)
> {
> + /* The non-pp refcount of net_iov is always 1. On net_iov, we only
> + * support pp refcounting which uses the pp_ref_count field.
> + */
> + if (netmem_is_net_iov(netmem))
> + return 1;
> +
> return page_ref_count(netmem_to_page(netmem));
> }
>
> static inline unsigned long netmem_to_pfn(netmem_ref netmem)
> {
> + if (netmem_is_net_iov(netmem))
> + return 0;
> +
> return page_to_pfn(netmem_to_page(netmem));
> }
>
> +static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> +{
> + return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> +}
> +
> +static inline unsigned long netmem_get_pp_magic(netmem_ref netmem)
> +{
> + return __netmem_clear_lsb(netmem)->pp_magic;
> +}
> +
> +static inline void netmem_or_pp_magic(netmem_ref netmem, unsigned long pp_magic)
> +{
> + __netmem_clear_lsb(netmem)->pp_magic |= pp_magic;
> +}
> +
> +static inline void netmem_clear_pp_magic(netmem_ref netmem)
> +{
> + __netmem_clear_lsb(netmem)->pp_magic = 0;
> +}
> +
> +static inline struct page_pool *netmem_get_pp(netmem_ref netmem)
> +{
> + return __netmem_clear_lsb(netmem)->pp;
> +}
> +
> +static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
> +{
> + __netmem_clear_lsb(netmem)->pp = pool;
> +}
> +
> +static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
> +{
> + return __netmem_clear_lsb(netmem)->dma_addr;
> +}
> +
> +static inline void netmem_set_dma_addr(netmem_ref netmem,
> + unsigned long dma_addr)
> +{
> + __netmem_clear_lsb(netmem)->dma_addr = dma_addr;
> +}
> +
> +static inline atomic_long_t *netmem_get_pp_ref_count_ref(netmem_ref netmem)
> +{
> + return &__netmem_clear_lsb(netmem)->pp_ref_count;
> +}
> +
> +static inline bool netmem_is_pref_nid(netmem_ref netmem, int pref_nid)
> +{
> + /* Assume net_iov are on the preferred node without actually
> + * checking...
> + *
> + * This check is only used to check for recycling memory in the page
> + * pool's fast paths. Currently the only implementation of net_iov
> + * is dmabuf device memory. It's a deliberate decision by the user to
> + * bind a certain dmabuf to a certain netdev, and the netdev rx queue
> + * would not be able to reallocate memory from another dmabuf that
> + * exists on the preferred node, so, this check doesn't make much sense
> + * in this case. Assume all net_iovs can be recycled for now.
> + */
> + if (netmem_is_net_iov(netmem))
> + return true;
> +
> + return page_to_nid(netmem_to_page(netmem)) == pref_nid;
> +}
> +
> static inline netmem_ref netmem_compound_head(netmem_ref netmem)
> {
> + /* niov are never compounded */
> + if (netmem_is_net_iov(netmem))
> + return netmem;
> +
> return page_to_netmem(compound_head(netmem_to_page(netmem)));
> }
>
> +static inline void *netmem_address(netmem_ref netmem)
> +{
> + if (netmem_is_net_iov(netmem))
> + return NULL;
> +
> + return page_address(netmem_to_page(netmem));
> +}
> +
> #endif /* _NET_NETMEM_H */
> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> index 61814f91a458..c6a55eddefae 100644
> --- a/include/net/page_pool/helpers.h
> +++ b/include/net/page_pool/helpers.h
> @@ -215,7 +215,7 @@ inline enum dma_data_direction page_pool_get_dma_dir(struct page_pool *pool)
>
> static inline void page_pool_fragment_netmem(netmem_ref netmem, long nr)
> {
> - atomic_long_set(&netmem_to_page(netmem)->pp_ref_count, nr);
> + atomic_long_set(netmem_get_pp_ref_count_ref(netmem), nr);
> }
>
> /**
> @@ -243,7 +243,7 @@ static inline void page_pool_fragment_page(struct page *page, long nr)
>
> static inline long page_pool_unref_netmem(netmem_ref netmem, long nr)
> {
> - struct page *page = netmem_to_page(netmem);
> + atomic_long_t *pp_ref_count = netmem_get_pp_ref_count_ref(netmem);
> long ret;
>
> /* If nr == pp_ref_count then we have cleared all remaining
> @@ -260,19 +260,19 @@ static inline long page_pool_unref_netmem(netmem_ref netmem, long nr)
> * initially, and only overwrite it when the page is partitioned into
> * more than one piece.
> */
> - if (atomic_long_read(&page->pp_ref_count) == nr) {
> + if (atomic_long_read(pp_ref_count) == nr) {
> /* As we have ensured nr is always one for constant case using
> * the BUILD_BUG_ON(), only need to handle the non-constant case
> * here for pp_ref_count draining, which is a rare case.
> */
> BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
> if (!__builtin_constant_p(nr))
> - atomic_long_set(&page->pp_ref_count, 1);
> + atomic_long_set(pp_ref_count, 1);
>
> return 0;
> }
>
> - ret = atomic_long_sub_return(nr, &page->pp_ref_count);
> + ret = atomic_long_sub_return(nr, pp_ref_count);
> WARN_ON(ret < 0);
>
> /* We are the last user here too, reset pp_ref_count back to 1 to
> @@ -281,7 +281,7 @@ static inline long page_pool_unref_netmem(netmem_ref netmem, long nr)
> * page_pool_unref_page() currently.
> */
> if (unlikely(!ret))
> - atomic_long_set(&page->pp_ref_count, 1);
> + atomic_long_set(pp_ref_count, 1);
>
> return ret;
> }
> @@ -400,9 +400,7 @@ static inline void page_pool_free_va(struct page_pool *pool, void *va,
>
> static inline dma_addr_t page_pool_get_dma_addr_netmem(netmem_ref netmem)
> {
> - struct page *page = netmem_to_page(netmem);
> -
> - dma_addr_t ret = page->dma_addr;
> + dma_addr_t ret = netmem_get_dma_addr(netmem);
>
> if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA)
> ret <<= PAGE_SHIFT;
> @@ -425,18 +423,17 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> static inline bool page_pool_set_dma_addr_netmem(netmem_ref netmem,
> dma_addr_t addr)
> {
> - struct page *page = netmem_to_page(netmem);
> -
> if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA) {
> - page->dma_addr = addr >> PAGE_SHIFT;
> + netmem_set_dma_addr(netmem, addr >> PAGE_SHIFT);
>
> /* We assume page alignment to shave off bottom bits,
> * if this "compression" doesn't work we need to drop.
> */
> - return addr != (dma_addr_t)page->dma_addr << PAGE_SHIFT;
> + return addr != (dma_addr_t)netmem_get_dma_addr(netmem)
> + << PAGE_SHIFT;
> }
>
> - page->dma_addr = addr;
> + netmem_set_dma_addr(netmem, addr);
> return false;
> }
>
> diff --git a/include/net/page_pool/types.h b/include/net/page_pool/types.h
> index 68a24c5ae827..e29e77f7934e 100644
> --- a/include/net/page_pool/types.h
> +++ b/include/net/page_pool/types.h
> @@ -6,6 +6,7 @@
> #include <linux/dma-direction.h>
> #include <linux/ptr_ring.h>
> #include <linux/types.h>
> +#include <net/netmem.h>
>
> #define PP_FLAG_DMA_MAP BIT(0) /* Should page_pool do the DMA
> * map/unmap
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 24d5236b2efc..22e3d439da18 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -25,7 +25,7 @@
>
> #include "page_pool_priv.h"
>
> -static DEFINE_STATIC_KEY_FALSE(page_pool_mem_providers);
> +DEFINE_STATIC_KEY_FALSE(page_pool_mem_providers);
>
> #define DEFER_TIME (msecs_to_jiffies(1000))
> #define DEFER_WARN_INTERVAL (60 * HZ)
> @@ -359,7 +359,7 @@ static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
> if (unlikely(!netmem))
> break;
>
> - if (likely(page_to_nid(netmem_to_page(netmem)) == pref_nid)) {
> + if (likely(netmem_is_pref_nid(netmem, pref_nid))) {
> pool->alloc.cache[pool->alloc.count++] = netmem;
> } else {
> /* NUMA mismatch;
> @@ -446,10 +446,8 @@ static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem)
>
> static void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
> {
> - struct page *page = netmem_to_page(netmem);
> -
> - page->pp = pool;
> - page->pp_magic |= PP_SIGNATURE;
> + netmem_set_pp(netmem, pool);
> + netmem_or_pp_magic(netmem, PP_SIGNATURE);
>
> /* Ensuring all pages have been split into one fragment initially:
> * page_pool_set_pp_info() is only called once for every page when it
> @@ -464,10 +462,8 @@ static void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
>
> static void page_pool_clear_pp_info(netmem_ref netmem)
> {
> - struct page *page = netmem_to_page(netmem);
> -
> - page->pp_magic = 0;
> - page->pp = NULL;
> + netmem_clear_pp_magic(netmem);
> + netmem_set_pp(netmem, NULL);
> }
>
> static struct page *__page_pool_alloc_page_order(struct page_pool *pool,
> @@ -695,8 +691,9 @@ static bool page_pool_recycle_in_cache(netmem_ref netmem,
>
> static bool __page_pool_page_can_be_recycled(netmem_ref netmem)
> {
> - return page_ref_count(netmem_to_page(netmem)) == 1 &&
> - !page_is_pfmemalloc(netmem_to_page(netmem));
> + return netmem_is_net_iov(netmem) ||
> + (page_ref_count(netmem_to_page(netmem)) == 1 &&
> + !page_is_pfmemalloc(netmem_to_page(netmem)));
> }
>
> /* If the page refcnt == 1, this will try to recycle the page.
> @@ -718,7 +715,7 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem,
> * refcnt == 1 means page_pool owns page, and can recycle it.
> *
> * page is NOT reusable when allocated when system is under
> - * some pressure. (page_is_pfmemalloc)
> + * some pressure. (page_pool_page_is_pfmemalloc)
> */
> if (likely(__page_pool_page_can_be_recycled(netmem))) {
> /* Read barrier done in page_ref_count / READ_ONCE */
> @@ -734,6 +731,7 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem,
> /* Page found as candidate for recycling */
> return netmem;
> }
> +
> /* Fallback/non-XDP mode: API user have elevated refcnt.
> *
> * Many drivers split up the page into fragments, and some
> @@ -928,7 +926,7 @@ static void page_pool_empty_ring(struct page_pool *pool)
> /* Empty recycle ring */
> while ((netmem = (__force netmem_ref)ptr_ring_consume_bh(&pool->ring))) {
> /* Verify the refcnt invariant of cached pages */
> - if (!(page_ref_count(netmem_to_page(netmem)) == 1))
> + if (!(netmem_ref_count(netmem) == 1))
> pr_crit("%s() page_pool refcnt %d violation\n",
> __func__, netmem_ref_count(netmem));
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index e1118b637085..cf23392e97f5 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -908,9 +908,9 @@ static void skb_clone_fraglist(struct sk_buff *skb)
> skb_get(list);
> }
>
> -static bool is_pp_page(struct page *page)
> +static bool is_pp_netmem(netmem_ref netmem)
> {
> - return (page->pp_magic & ~0x3UL) == PP_SIGNATURE;
> + return (netmem_get_pp_magic(netmem) & ~0x3UL) == PP_SIGNATURE;
> }
>
> int skb_pp_cow_data(struct page_pool *pool, struct sk_buff **pskb,
> @@ -1008,11 +1008,10 @@ EXPORT_SYMBOL(skb_cow_data_for_xdp);
> #if IS_ENABLED(CONFIG_PAGE_POOL)
> bool napi_pp_put_page(netmem_ref netmem, bool napi_safe)
> {
> - struct page *page = netmem_to_page(netmem);
> bool allow_direct = false;
> struct page_pool *pp;
>
> - page = compound_head(page);
> + netmem = netmem_compound_head(netmem);
>
> /* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation
> * in order to preserve any existing bits, such as bit 0 for the
> @@ -1021,10 +1020,10 @@ bool napi_pp_put_page(netmem_ref netmem, bool napi_safe)
> * and page_is_pfmemalloc() is checked in __page_pool_put_page()
> * to avoid recycling the pfmemalloc page.
> */
> - if (unlikely(!is_pp_page(page)))
> + if (unlikely(!is_pp_netmem(netmem)))
> return false;
>
> - pp = page->pp;
> + pp = netmem_get_pp(netmem);
>
> /* Allow direct recycle if we have reasons to believe that we are
> * in the same context as the consumer would run, so there's
> @@ -1045,7 +1044,7 @@ bool napi_pp_put_page(netmem_ref netmem, bool napi_safe)
> * The page will be returned to the pool here regardless of the
> * 'flipped' fragment being in use or not.
> */
> - page_pool_put_full_netmem(pp, page_to_netmem(page), allow_direct);
> + page_pool_put_full_netmem(pp, netmem, allow_direct);
>
> return true;
> }
> @@ -1072,7 +1071,7 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
> static int skb_pp_frag_ref(struct sk_buff *skb)
> {
> struct skb_shared_info *shinfo;
> - struct page *head_page;
> + netmem_ref head_netmem;
> int i;
>
> if (!skb->pp_recycle)
> @@ -1081,11 +1080,11 @@ static int skb_pp_frag_ref(struct sk_buff *skb)
> shinfo = skb_shinfo(skb);
>
> for (i = 0; i < shinfo->nr_frags; i++) {
> - head_page = compound_head(skb_frag_page(&shinfo->frags[i]));
> - if (likely(is_pp_page(head_page)))
> - page_pool_ref_page(head_page);
> + head_netmem = netmem_compound_head(shinfo->frags[i].netmem);
> + if (likely(is_pp_netmem(head_netmem)))
> + page_pool_ref_netmem(head_netmem);
> else
> - page_ref_inc(head_page);
> + page_ref_inc(netmem_to_page(head_netmem));
> }
> return 0;
> }
> --
> 2.44.0.rc1.240.g4c46232300-goog
>
--
Thanks,
Mina